1. Field of the Invention
This invention relates to electronic semiconductor devices, their use, and method of preparation. More particularly, this invention relates to a material useful for an electrical contact with a semiconductor material.
2. Description of the Prior Art
In the manufacture of electronic semiconductor devices, it is necessary to contact materials to semiconductors that form both rectifying and non-rectifying junctions. The active region of the device may be a specific arrangement of contacts that form rectifying and non-rectifying junctions, while the interconnect contacts are generally junctions that are non-rectifying in nature.
Contacts that form rectifying junctions through which electrical current flows in one direction against very low resistance and against a very high resistance in the reverse direction are typically formed between two semiconductors, between a metal and a semiconductor, or between two doped regions of a semiconductor. Contacts that form non-rectifying junctions through which electrical current flows against low resistance in both directions may also be formed between metals and/or semiconductors. A contact forming a non-rectifying junction has a resistance relative to the bulk resistance of the semiconductor such that the contact does not significantly perturb device performance, and it can supply the required current with a voltage drop that is sufficiently small compared with the drop across the active region of the device. In contrast, a contact forming a rectifying junction has a high resistance relative to the bulk resistance of the semiconductor, and thus provides a low reverse saturation current.
A contact forming a rectifying junction is often formed in a homojunction device by doping a semiconductor like silicon so that a p-n junction is formed. The dopants are generally thermally diffused into the semiconductor. The high process temperatures necessary to effect diffusion are often detrimental to devices such as very large scale integrated circuits (VLSI). Contacts forming rectifying junctions can also be formed at lower temperatures at metal-semiconductor Schottky-barriers of heterojunction semiconductor-semiconductor contacts. In all cases, including both the hetero- and homojunction cases, the rectifying property is induced by the energetic mismatch of the Fermi levels of the two materials. A quantitative measure of this mismatch is the barrier height. The larger the barrier height, the lower the reverse saturation current.
In metal-semiconductor Shottky-barrier devices based on n-silicon, metals with large work functions contact semiconductors to form highly rectifying junctions. (As used herein, the work function of a metal is the minimum energy necessary for an electron of the metal to escape into vacuum from an initial energy at the Fermi level of the metal.) The largest barrier height observed with n-silicon is approximately 0.8-0.9 V (S. M. Sze, Physics of Semiconductor Devices, Second Edition, Wiley, N.Y., 1981, herein known as "Sze," p. 291). The sum of the barrier heights for a given metal film on n- and p-type semiconductors theoretically equals the band gap of the semiconductor. However, surface states that form during device fabrication often result in reduced barrier heights due to Fermi-level pinning. Such pinning can usually be eliminated by growing a film of insulating material (e.g., SiO.sub.x) of tunneling dimensions (e.g. 10-20 Angstroms) between the substrate semiconductor and the coating material.
The search continues for materials used to effectively form rectifying and non-rectifying junctions with semiconductors. Accordingly, in the case of contacts forming rectifying junctions, it is desirable to form a junction with a very large barrier height and a low number of surface states, while a junction with a low barrier height is desirable for contacts forming non-rectifying junctions. It is also desirable to form such junctions at low temperatures.
It is therefore a major object of this invention to provide an electronic semiconductor device having a contact that forms an effective rectifying (high barrier height) and/or non-rectifying junction between semiconductor materials.
Another object of this invention is to provide an electronic semiconductor device having contacts forming rectifying and/or non-rectifying junctions prepared at relatively low temperatures.
These and other objects and advantages of this invention will become apparent from the following description.